The present invention relates to an improved brake shoe for use in a railway braking system. More particularly, a composition brake shoe is provided with a bidirectional laminated structure, which shoe is mountable on a metal backing plate affixable to a brake head or operator. The shoe has a high-friction composition material for wheel tread gripping, and a low-friction material for contact with the wheel flange to minimize the wear and heat buildup in the flange. Frictional contact between a high-friction-composition brake shoe element and the railroad wheel flange may create temperature gradients in the wheel flange which can induce residual tensile stresses. Lower thermal effects reduce the propensity for development of undesired residual tensile stresses in the wheel flange, which tensile stresses can lead to catastrophic wheel failure. Brake shoes on railway vehicles, that is cars and locomotives, have been utilized since the inception of the railway industry. The brake shoe for a considerable period of time in history was a cast iron body having a substantial thickness and a curved braking surface generally corresponding to the curvature of a railway car wheel. The non-contacting or back-side of these cast iron brake shoes frequently included a supporting plate or strip formed from rolled steel around which the brake shoe body was cast. The supporting plate or strip reinforced the cast iron body and acted to maintain its structural integrity in the event of brake shoe fracture during service.
A second type of brake shoe currently in use on railway cars and locomotives has an organic-based composition body supported on a steel backing plate, which backing plate may be similar to the support member of the above-noted cast iron brake shoe. An exemplary railway brake shoe arrangement is illustrated in U.S. Pat. No. 4,466,513 to Dedek. In composition brake shoes, the key lugs, rejection lugs, and other elements of the brake shoe assembly on the mounting plate and brake head, are formed as part of the supporting backing-plate. The above-noted cast iron shoe structure may also have key lugs, toe guides and various other elements cast as part of the brake shoe body.
The composition type brake shoes generally have a substantially higher retarding force per unit of brake-applied force than the cast-iron brake shoes, as they have a higher coefficient of friction than is obtainable with a cast-iron brake shoe. The composition shoes are noted as high-friction, low-pressure devices, as they may be utilized at substantially lower operating pressures than the cast iron shoe, while cast iron shoes are frequently referred to as low-friction, high-pressure devices.
High friction brake shoe compositions at normal operating conditions are or may usually be characterized by a dynamic coefficient of friction between about 0.25 to 0.50, however, under known specific circumstances the coefficient of friction may be outside this range. Similarly, low friction brake shoes may broadly be considered to have dynamic coefficients of friction between about 0.15 to 0.30. These ranges for the coefficients overlap at their respective lower and upper bounds. However, the differential between the material coefficients illustrates the difference in the materials and distinguishes their applicability to a particular function.
A particular difficulty associated with the composition brake shoes had been their tendency to break or fracture under severe operating conditions and to separate from the metal backing plate even under mild braking conditions. These disadvantages may have been attributable to the difference in thermal coefficients of expansion of the composition body and the metal back structure, which may have resulted in the brake shoe body being stressed beyond the fracture point or inducing separation from the metal backing plate from flexural forces on the brake shoe. The relatively severe vibrational forces encountered in railway service and the shock loading on the brake shoe upon initial application of the brakes to the wheel may also act to detach the composition body from the metal back and/or to fracture the composition brake shoe body.
Illustrative brake shoe body compositions are taught in U.S. Pat. No. 3,168,487--Spokes, et al, and U.S. Pat. No. 3,227,249--Kuzmick, which latter patent teaches a composition brake shoe with an organic bond matrix, a hard mineral filler and a cryolite addition to reduce the wear of the contacted wheel. Inorganic fillers are also added to shoe compositions to stiffen or reinforce the organic bond.
U.S. Pat. No. 4,219,452--Littlefield discloses the preparation and manufacture of a high friction composition railroad brake shoe, which composition is devoid of asbestos and is operable to withstand braking parameters associated with the deceleration of railroad locomotives. This composite element has a rubber and/or resin binder with a plurality of fillers, at least one of which has an oil absorption value of at least 30, and a fiber formed from an aramid polymer. More specifically, the asbestos free composite friction element incorporates a curable rubber binder; hard mineral fillers; friction modifiers; reinforcing aramid fibers; and the above-noted absorptive fillers. The asbestos-free composite friction shoe is capable of withstanding high temperatures, has high physical strength, and is operable to provide braking characteristics to meet the test standards of the Association of American Railroads (AAR) for brake shoes made with a blend of organics and/or inorganic materials. This standard is noted at Specification M-926 in the Manual of Standards and Recommended Practices from the Association of American Railroads-Mechanical Division. Each of the above-noted composite materials is referred to as a high-friction component, which are elements tolerant of high temperatures and require only low pressure to apply their braking and gripping force to railroad wheels.
At braking of a moving rail car, the change in potential and kinetic energy is converted to heat at the brake shoe-railroad wheel interface. It is desirable to provide the wheel shoe interface at the wheel tread surface and not the flange, as contact between the high friction composition brake shoes and the flange can lead to excessive heating of the flange from the frictional engagement between the flange and the high-friction brake shoe.
Railroad car brake shoes during braking engagement with the wheel are susceptible to displacement from their position relative to the wheel tread. The brake shoe may migrate laterally and engage the wheel tread with less than all of its facing or braking surface, and further may engage the flange of the wheel. These migrating conditions are usually parallel to the wheel and axle axis, which produces an overhanging condition, that is brake shoe migration off the wheel tread, or an overriding condition where the brake shoe edge contacts and wears on the wheel flange. As the brake shoes on an axle are frequently coupled by a connecting beam, lateral migration by one of the shoes to an overriding or overhanging condition induces the alternate condition in the opposite brake shoe. In the overriding shoe position, the rubbing contact of the brake shoe with a wheel flange can produce residual tensile stresses in the flange area, which may lead to the propagation of cracks, catastrophic wheel failure and subsequent broken-wheel derailments. Discussions, analysis and evaluation of the effects of residual stresses, cracks, fatigue and other variables on railroad wheel failures and fractures are provided in the following articles:
1.) "Modeling the Drag Braking of Freight Car Wheels Having A Simulated Heat Treatment" by A. J. Opinsky and M. W. Joerms;
2.) "An Interpretive Review of Wheel Failure Performance With Respect to Design and Heat-Treatment" by D. H. Stone;
3.) "North American Wheel Failure Experience" by D. H. Stone, W. S. Pellini and W. J. Harris, Jr.; and
4.) "Safe Thermal Loads for a 33-inch Railroad Wheel" by H. R. Wetenkamp and R. M. Kipp.
Although it is desirable to provide a brake apparatus, which is perfectly stable and consistently alignable with the tread of the railroad wheel, the record of railroad wheel accidents and derailments illustrates that this desire has not yet been attained. Thus, there are continuing efforts to produce new brake apparatus and to reduce the effects, if not the fact, of contact between the railway brake shoe and the railway-wheel flange. Resolution of the wheel flange-brake shoe contact problem requires balancing the need to provide adequate braking forces on the wheels by providing maximum surface contact between the brake-shoe-tread-contacting-surface and the wheel tread while minimizing or eliminating contact between the brake shoe and the flange. In an ideal situation, the brake head mechanism would maintain the brake shoe body in alignment with the wheel tread face. As new developments continue on the brake head alignment problem, other research developments are being directed to minimizing the effects of brake shoe migration, brake shoe fracture, tread wear, flange engagement, flange wear, and catastrophic wheel failure.
The brake shoe of the present invention does not detract from the gripping or tread-contacting surface of the high friction shoe element, but inhibits and minimizes the effect of contact between the brake shoe assembly and the wheel flange. Thus, the incidence or occurrence of excessively high temperatures in the flange from frictional contact between the brake shoe high friction material and the wheel flange is minimized, which results in lower thermal stresses in the wheel flange. Concomitantly, the incidence of residual tensile stresses in the flange area from intimate contact between this high friction brake shoe material and the wheel flange, and the consequent fatigue cracking, stress fractures and potential derailments resulting therefrom, are also expected to be reduced.